1. Field of the Invention
The present invention relates to a high-frequency oscillator, and more particularly, to a high-frequency oscillator of a frequency switching type(hereinafter referred to as an oscillator of a frequency switching type), which can switch among a plurality of high frequencies and output a signal.
2. Description of the Related Art
A high-frequency oscillator that outputs a 600-MHz or higher frequency by using a crystal resonator with high stability is used as a frequency source, etc. of an optical communication. The high-frequency oscillator normally amplifies the output of a crystal oscillator with frequency multiplication, for example, by using a plurality of LC filter stages and an amplifier, and obtains a target high-frequency output.
In recent years, instead of this configuration, a high-frequency oscillator having a configuration where a high-frequency output is obtained by distorting the output of a crystal oscillator, and by selecting a harmonic component with a SAW (Surface Acoustic Wave) filter has been implemented, for example, as recited in Japanese Patent Application No. 2000-244682. With this configuration, the number of elements is decreased, so that the outer dimensions of the oscillator are significantly reduced.
FIG. 1 is a circuit diagram of a high-frequency oscillator having such a configuration.
The high-frequency oscillator shown in this figure is configured by a crystal oscillator 1, a SAW filter 2, and an amplifier 3. The crystal oscillator 1 is composed of a crystal unit 4 which is, for example, AT-cut, a split capacitor which is not shown and forms a resonant circuit along with the crystal unit 4, and an oscillation amplifier 5 which feeds back and amplifies a resonant frequency. Here, what is called an oscillation circuit of a colpitts type is formed.
The SAW filter 2 is configured by forming, for example, input/output interdigital transducers (IDTs) on a piezoelectric substrate not shown. The SAW filter 2 makes only a component in a particular frequency region of an input signal pass through, and outputs the component.
The amplifier 3 is implemented as a broadband amplifier 3 such as a linear IC amplifier, etc. With the amplifier 3, the output of the SAW filter 2 is amplified with a linear portion, namely, a non-saturation portion of the input/output characteristic of the broadband amplifier 3, so that a high-frequency output is obtained.
The crystal oscillator 1 referred to here is a voltage controlled type implemented by inserting a voltage variable capacitance element 6 in an oscillation closed loop of the crystal oscillator 1. The oscillation frequency of the crystal oscillator 1 is varied by a control voltage Vc that is applied via a high-frequency blocking resistor 7. Additionally, Vcc in FIG. 1 is a power source voltage. Generally, a complementary-output driver IC 8 is connected to the amplifier 3.
In the high-frequency oscillator shown in FIG. 1, the oscillation output of the crystal oscillator 1 is distorted. For example, the top of a sinusoidal wave is cut and distorted to be rectangular as shown in FIG. 2 by making a center voltage Voo of the oscillation output higher than a center voltage Vco of the power source voltage Vcc. The oscillation output of the crystal oscillator 1 is distorted in this way, so that the levels of harmonic components f2 through fn, relative to a fundamental component f1 within a frequency spectrum, are raised in the oscillation output as shown in FIG. 3. In FIG. 2, only the top of the sinusoidal wave is distorted to be rectangular. However, the top and the bottom of the sinusoidal wave may be distorted to be rectangular, for example, by increasing the amplification rate of the oscillation amplifier 5.
The respective components of the frequency spectrum are made equal to or higher than a predetermined level as shown in FIG. 3, whereby an arbitrary harmonic component can be selected by inputting this oscillation output to the SAW filter 2. For example, the fundamental frequency (fundamental component) f1 of the crystal oscillator 1 is implemented to be 155.52 MHz, which is almost as high as the manufacturing limit of the crystal oscillator 1, and a harmonic component 622.08 MHz, which becomes a quadruple-frequency wave f4, is selected with the SAW filter 2, and amplified with the amplifier 3, so that a high-frequency output is obtained. If the oscillation output of the crystal oscillator 1 is not distorted, the level of a harmonic component relative to the fundamental component is low. Therefore, the harmonic component cannot be selected with the SAW filter 2.
However, with the high-frequency oscillator having the above described configuration, only a signal of one high frequency, for example, only a signal of 622.08 MHz can be output. Accordingly, if a communications appliance requires, for example, signals of 2 frequencies such as 622.08 MHz and 666.5143 MHz, two high-frequency oscillators, each of which comprises a crystal oscillator 1, a SAW filter 2, and an amplifier 3 must be mounted on a set substrate for each of the frequencies, and an output signal from either of the oscillators must be selected and used. Accordingly, the configuration of the high-frequency oscillator becomes large in this case, leading to an increase in the cost, and an obstacle to the downsizing of the set substrate.
An object of the present invention is to provide an oscillator of a frequency switching type, which can select and output any of a plurality of high frequencies, and can reduce a device in size.
An oscillator of a frequency switching type according to the present invention comprises a plurality of crystal oscillation units, a power switching unit, and a SAW filter unit.
The plurality of crystal oscillation units respectively have oscillation outputs whose fundamental frequencies are different.
The power switching unit operates one of the plurality of crystal oscillation units.
The SAW filter unit is implemented by forming a plurality of IDTs (Inter Digital Transducers), which set passbands of different frequencies, on the same piezoelectric substrate. To the SAW filter, the above described oscillation output of the crystal oscillation unit that is operated by the power switching unit is input.
An oscillator of a frequency switching type having another configuration according to the present invention comprises a plurality of crystal oscillation units having oscillation outputs whose fundamental frequencies are different, a plurality of filter units, which are respectively arranged for the plurality of crystal oscillation units and whose frequency passbands are different, and an output frequency selection unit selecting a frequency of a high-frequency output that is output from the high-frequency oscillator.
The plurality of filter units is a SAW filter which is configured by forming a plurality of IDTs, which set, by way of example, passpands of different frequencies, on the same piezoelectric substrate.
The output frequency selection unit selects the frequency a high-frequency output, which is ouput from the high-frequency oscillator, by supplying power to one of the plurality of crystal oscillation units.
The plurality of crystal oscillation units distort and output the oscillation output.
An oscillator of a frequency switching type having a further configuration according to the present invention comprises a crystal oscillation unit distorting and outputting an oscillation output, a plurality of filter units whose frequency passbands are different, and an input selection unit selecting one of the plurality of filter units, and inputting the oscillation outputs.
According to the present invention, a plurality of high-frequency signals can be selected and output. Additionally, a device can be downsized by reducing a plurality of filters in size.